1. Field of the Invention
The present invention generally relates to a radar apparatus for an automobile. More specifically, the present invention is directed to an automobile radar apparatus suitable for performing automatic vehicle tracking control to automatically hold a predetermined safety distance between successive driving vehicles by detecting both a distance and a direction to the preceding vehicle.
2. Description of the Prior Art
Since the number of automobiles is considerably increased, frequent occurrence of traffic jams on roads have also increased. In particular, such traffic jams very usually occur on the general-purpose roads and highways in large cities and towns around large cities. Consequently, stress to drivers who drive on such roads crowded with automobiles has also considerably increased. Such stress leads to an increased occurrence frequencies of traffic accidents due to hampered judgement caused by driver's wearing. To mechanically and electronically avoid automobile such as these collisions various types of safety apparatus are required such as an automatic inter-vehicle distance maintaining apparatus, a inter-vehicle distance alarm apparatus, and an automatic braking apparatus. Furthermore, developments in automatic preceding-vehicle tracking apparatus can considerably reduce such stress on drivers. To effectively achieve the functions of these apparatus, an automobile radar apparatus capable of sensing a direction toward and a distance between a preceding vehicle and a self-vehicle, is necessarily required.
Conventionally, as a method for detecting the distance between a preceding vehicle and a self-vehicle, several systems are well known. That is to say, a distance is measured by calculating a transmission time for either electromagnetic waves, or optical pulses projected to the preceding vehicle. However, such a conventional vehicle-distance detecting system cannot be relied upon always to detect a correct distance because when the average vehicle distances are relatively short, as in the case of the above-described traffic jam, the transmission time becomes too short for accurate measurement. As another alternative distance detecting system for such a short distance, a range finder apparatus used for an automatic focusing operation of a camera can be used. As to such a range finder apparatus, a triangulation system is typically employed by way of either a passive method for utilizing incoming light, or an active method for emitting light from the distance measuring apparatus per se. Either passive method or active method can detect a distance, but can not detect a direction.
One conventional automobile radar apparatus has been proposed in, for instance, Japanese Laid-open patent application No. 63-120212, which utilizes the above-described triangulation system so as to detect both a distance and a direction to a preceding vehicle. More specifically, this radar apparatus detects heat emitted from an exhaust tube of the preceding vehicle. Unfortunately, this thermal detecting system cannot be applied to all vehicles, e.g., a motor truck equipped with an exhaust tube that is not at a rear side thereof, or a vehicle equipped with a plurality of exhaust tubes. Furthermore, such a system has the serious drawback that it is rather difficult to specify a heat source under high temperature conditions in summer even if a vehicle is equipped with a single exhaust tube at a rear side thereof. In addition, there are problems that the response characteristic of the detecting element for detecting such a low-temperature heat source is not so acceptable, and the element must be cooled. As a consequence, the radar apparatus becomes complex and expensive in cost.
To the contrary, another system has been opened in, for example, Japanese Laid-open patent application No. 49-43328, in which a light source is employed at a rear side of a preceding vehicle so as to project light rays therefrom toward a rear direction, and both a distance and a direction of the self-vehicle from a preceding vehicle are obtained by calculating a difference in the amount of light from a plurality of light receivers. Since distance and direction are calculated based upon the difference in the received light amounts of plural light receivers in the above-described conventional system, changes in the due to soiled light source and soiled light receivers, changes in the light emitting amounts of the light source due to temperature variations and aging, and also changes in the respective amounts of light received due to sensitivity changes of each of the respective light receivers, cannot be ignored. Moreover, since these changes in the received amounts of light are different from each other, there are other drawbacks to stable detecting operations of the distance and direction over a long time range.
FIG. 13 represents an arrangement of an automobile radar apparatus, in which a light emitting device is provided on a preceding vehicle, a self-vehicle is equipped with a rotatable optical system and a distance to from the self-vehicle preceding vehicle and the is detected.
It should be noted that this FIG. 13 corresponds to a block diagram for representing a fourth preferred embodiment of the present invention as will be discussed later. Nevertheless, reference is now made to FIG. 13 for showing a vehicle mounted radar device of the present invention in which a light emitting unit is mounted a preceding vehicle and an inter-vehicle distance is detected by a rotatable optical system on a self-vehicle constructed as shown in FIG. 13. In order to clarify the item to be targeted in reference to the second object, the present invention will be described in reference to FIGS. 13, 1 and 2, respectively.
In FIG. 13, reference numeral 1 denotes a light source which is installed at a predetermined position of a rear side (part) of a preceding vehicle 100. The light source 1 emits light in a blocking mode by way of a light source circuit 2.
Reference numeral 4 indicates a rotatable light-receiving optical system. A pair of the rotatable light-receiving optical systems are provided on a front side of a self-vehicle 200 in such a manner that two optical systems are separated from each other by a base length "B". These light-receiving optical systems 4 are separately driven by rotating (pivoting) means 10L and 10R. These rotating means 10L and 10R are under the control of rotation control means 9L and 9R, respectively.
In the respective light-receiving optical systems 4, there are provided light position detectors 42L and 42R. These light position detectors 42L and 42R are positioned in such a manner that the light emitted from the light source 1 is focused onto a light receiving plane of the respective detectors via light-receiving lens 41L and 41R.
Signals derived from the light position detectors 42L and 42R are processed by optical position process means 6L and 6R, respectively, so as to output a light position signal representative of an incident position of light, and a light receiving signal indicating that intermittent light is incident.
When the outputs from the light position process means 6L and 6R are supplied, an optical system rotation control means 21 outputs a signal to the rotation control means 9L and 9R so as to drive the rotation means 10L and 10R, whereby the rotation of the light-receiving optical system 4 is controlled.
Turning angle detecting sensors 11L and 11R detect an angle defined by a wheel shaft of the self-vehicle 200 and an optical axis of the light-receiving optical system 4, and output the detected angle signals to a control means 20.
In response to the outputs from the turning angle detecting sensors 11L and 11R, the control means 20 calculates a vehicle distance "L" between the preceding vehicle 100 and the self-vehicle 200, and also a direction ".theta." of the preceding vehicle 100.
It should be noted that both the optical system rotation control means 21 and control means 20 may be united as a computer 25.
Referring now to flowcharts shown in FIGS. 1a and 1b, operations will be described. In FIG. 1a, when a main routine starts (step S1), CPU (computer unit 25) is initialized at a step S2.
Next, at a step S3, a flag check is made, and waits until the flag becomes true. This flag becomes true by way of a process of an interrupt routine.
That is to say, when both of the light receiving signals are input into the computer 25, the program control is moved to the interrupt routine.
Once the interrupt operation starts at a step S8 represented in FIG. 1b, first of all the light position signal is read and the following calculations are executed at a step 9. That is to say, as indicated in FIG. 2, angles .DELTA..phi..sub.L and .DELTA..phi..sub.R defined between the optical axes 5 of the light-receiving lens 41L and 41R and a line for connecting the light source 1 and a principal point 40 of the light-receiving lens 41L and 41R, are calculated based on a calculation formula: .DELTA..phi.=tan.sup.-1 (.DELTA.P/F), when "F" indicates a distance between the principal point 40 and the optical position detectors 42L and 42R, and ".DELTA.P" denotes a shift between the principal point 40 of the lens and light source 1 in the light position detectors 42L and 42R.
Subsequently, at a step S10, the optical system rotation control means 21 drives the drive means 10L, 10R via the rotation control means 9L, 9R so as to rotate the light-receiving optical system 4, respectively.
In other words, this interrupt routine is to control the turning angle of the light-receiving optical system 4 in such a manner that the optical axes of the light-receiving lenses 41L and 41R are coincident with the line for connecting the light source 1 and the principal point 40 of light-receiving lenses 41L and 41R.
Finally, the turning angle of the light-receiving optical system 4 sets the flag indicating that the axes of the light-receiving lens 41L and 41R are coincident with the line for connecting the light source 1 and the principal point 40 of the light-receiving lens 41L and 41R, to true (at a step S11), whereby the interrupt routine is ended (at a step S12).
As previously described, when the flag is set to true, according to the main routine, the turning angles .phi..sub.L, .phi..sub.R (FIG. 13) of the light-receiving optical system 4 is read at the step S4 shown in FIG. 1a by the turning angle sensors 11L and 11R, the vehicle distance L and direction .theta. are calculated at a step S6 based upon equations (3), (4) (will be described later), and the vehicle distance L and direction .theta. are output at the step S6.
Finally, at the step S7, the flag is cleared (FAIL) and the process is returned to the flag check operation at the step S3, and the subsequent interrupt routine process is accomplished and waits until the flag becomes true.
In the above described detections of such parameters as the inter-vehicle distance, in case that there is a clear light/dark distribution in a background light, when the light-receiving optical system 4 is rotated by the turning angle of .DELTA..phi. in conjunction with the movement of the light source 1, the light/dark distribution of the background light is accordingly varied.
Now considering arbitrary positions on the light position detectors 42L and 42R, the light incident upon these positions becomes an intermittent mode due to the variation of the light/dark distribution of the background light.
As a consequence, no discrimination can be made that this intermittent light comes from the light source 1 equipped with the preceding vehicle 100, or the variations of the background light caused by the rotation of the mirror. Then, in such a place that the light/dark distribution is present in the background light, it is difficult to detect a correct vehicle distance.
If the light-receiving optical system 4 is slowly rotated in order not to form the background light as the intermittent light, the time intervals to detect the vehicle distance "L" or the like are prolonged and therefore there is another problem that the response characteristics are deteriorated.
Also, another automatic tracking apparatus for automatically maintaining a safe vehicle distance between a preceding vehicle and a self-vehicle is disclosed in, for example, Japanese Laid-open patent application No. 60-19208 and No. 60-163732. These conventional apparatus are so constructed that both the throttle valve and brake are automatically actuated in response to outputs derived from the inter-vehicle sensing means so as to continuously keep the vehicle distance between the preceding vehicle and the self-vehicle at a safe inter-vehicle. Then, the vehicle distance sensing means projects either the electromagnetic wave or ultrasonic wave to the preceding vehicle, and receives the reflections from the preceding vehicle so as to sense the vehicle distance between the preceding and the self-vehicle based upon the time required for such a wave transmission and reception. As a consequence, although the distance between the preceding vehicle and tracking vehicle can be correctly detected, the positional shifts between the driving lanes of the preceding vehicle and tracking vehicle cannot be sensed.